The concentration of biologically significant substances in various liquids, such as blood, urine, fluids or others, is usually determined through two totally different methods. On the one hand through various determined (antigenic concentration). Such methods include made known (antigenic concentration); such methods include the radioimmunoassay (Travis, J. C., Clinical Radioimmunoassay, State of the Art, Radioassay, Ligand assay Publishers, Anaheim, Calif. 1980), the enzyme-linked-immunosorbent-assay (Nakamura, R. M., Dito, W. R. Tucker II E. S., Immunoassays in the Clinical Laboratory, Laboratory and Research Methods in Biology and Medicine, Vol. 3, Alan A. Liss Inc., New York, 1979; Voller, A., Bidwell, D. E., Bartlett, A., The Enzyme Linked Immunosorbent Assay - ELISA, The Authors, 1979, London; Langone,J. J., Van Vunakis, H., Methods in Enzymology, Immunochemical Techniques Part E, 1983, Academic Press; further Schuurs et al., U.S. Pat. No. 29,169 and Schuurs et al. , U.S. Pat. No. 4 016 043 )or coarse methods (Nakamura, R. M., Dito, W. R. Tucker III, E. S. Immunoassays in the Clinical Laboratory, Laboratory and Research Methods in Biology and Medicine, Vol. 3, Alan R. Liss Inc., New York, 1979; Langone, J. J., Van Vunakis, H., Methods in Enzymology, Immunochemical Techniques Part B, 1981, Academic Press), such as immunonephelometry, radial immunodiffusion or other appropriate immuno-precipitation processes. With the aid of immunological determination, it is possible to establish the concentration of a substance. However, it is impossible to draw any conclusion as to the function of this so-established substance. It has been proven frequently that, in various cases, in a normally immunologically determinable concentration, the function of this molecule did not correspond to the one immunologically determined and often was well below the normal value. As is the case with such molecules in proteins, having a precise enzymatic function or a carrier or cofactor function or the like in the body fluid and whose function is impaired by modifications in certain molecular fragments or which are inhibited by fixation to inhibitors.
The second possibility to establish the concentration of a certain substance in body fluids is the determination of the function of this substance (Bernmeyer, H. U., Methods of Enzymatic Analysis, Vol. 5 Verlan Chemie, 1984). Thereby, various systems are used, most of them being based on an enzyme reaction and which measure the enzymatic activity of the desired substance (Lorand, L., Proteolytic Enzymes, Methods in Enzymology, Vol. 45, 1976, Academic Press). By selecting specifically suitable substrates (e.g. high-molecular natural substrates, which are transformed by enzymes, such as fibrinogen for its enzyme thrombin, fibrin as substrate for plasmin, plasminogen as substrate for plasminogen activators, kininogen as substrate for kallikrein , but also low-molecular substrates, which contain the specific peptide compound for the respective enzyme and whose decomposition can lead, for instance, to a color reaction) and suitable determination criteria (thereby in the case of active enzyme formation, in the first stage the respectively formed enzymes are measured, respectively in the case of low-molecular substrates, the colored products, resp. newly formed or lost characteristics of substrates, such as the capability to form a bond with certain substances. An example of the los of such capability is the binding of the active plasminogen activator-inhibitor to its binding protein, whereby this binding capability is lost in the deactivation of the plasminogen activator-inhibitor. Further examples for these assaying processes involve binding to specific inhibitors, transfer proteins or other binding proteins, which can be correspondingly marked and be traced through a coupled reaction. It is possible to obtain a relatively high specificity for the determined substance; however, it is impossible to indicate a measured value with absolute specificity, since normally a multitude of enzymes or other substances catalyze the same or a similar reaction on the same substrates, although with different speeds. An example of same is the activation of plasminogen to plasmin through plasminogen activators, whereby at least two main plasminogen activators are present in the body fluid, each of them splitting the same bond in the plasminogen, whereby an identical plasmin is formed. Thus, with the functional method it is not possible to perform the quantitative analysis of a precisely defined substance; it is possible only to ascertain a functional defect of the entire system.